Of these, I have direct experience with Zope. Zope has the disadvantage that it would likely require some upfront customization to do just what is needed. On the other hand, the other's probably do as well, if they're customizable at all. If an out of the box solution is needed, that WorldOx, Intraspect, or DocuShare might be good candidates for further study.

I'm surprised that no one mentioned NextPage or Groove. NextPage may be out of reach of small businesses and Groove is seen more as a collaboration space. On the other hand, document management is partly about collaboration---that's why you need to manage documents most of the time.

This exercise started out from a question from a friend. I think I'll recommend he look at Zope first.